In recent years, a power generation system using renewable energy, such as solar energy and wind energy, is drawing attention. However, in the power generation system, the amount of power generation is significantly affected by the weather, environment, and the like, and stable supply of power to an existing power distribution network and consumers, such as households, is difficult to achieve. Therefore, a technique capable of stable supply of power to the power distribution network and the consumers by using the power generation system is being developed.
A technique of controlling output power that is output from a power supply source, such as a power generation apparatus like a solar panel and a power storage that accumulates power generated by the power generation apparatus, is proposed as a technique for stable supply of power. The technique allows stable supply of power by supplying power to the power distribution network with the output power of the power generation apparatus or the power storage held constant or by supplementing insufficient power with power from the power storage when the power demand of the consumers exceeds the current power generation amount.
To realize the technique for a stable supply of power, a power source that has a combination of a power supply source and a power converter that controls the output power of the power supply source is necessary.
It is desirable that the power converter included in the power source be compatible with a high voltage high current. However, there are limitations in the volume of a passive element used in the power converter, the heat radiation efficiency of a switching element, and the like, and it is difficult to realize the power converter corresponding to the high voltage high current.
Meanwhile, proposed is a method of constructing a power system, in which constant voltage power sources including power converters corresponding to a high voltage low current that can be easily realized are connected in parallel to thereby handle a high voltage high current.
However, when the constant voltage power sources are connected in parallel, there are minute differences between output voltages of the constant voltage power sources, and as a result, a theoretically infinite current called a cross current may be generated between the constant voltage power sources. When the cross current is generated, there are problems in which the output currents of the constant voltage power sources become unbalanced, the output voltages of the constant voltage power sources become unstable, and the lifetime of the constant voltage power source is reduced.
A method of connecting resistors to connection terminals of the constant voltage power sources to bring the output voltages of the constant voltage power sources into line (see Patent Literature 1) and a method of connecting diodes to the connection terminals to prevent cross current are proposed to solve the problems. However, in the methods, the registers and the diodes connected to the connection terminals of the constant voltage power sources cause heat loss in which power is converted to heat, and the efficiency of supplying power is reduced. Although a method of controlling the power converters in the constant voltage power sources in a coordinated manner to bring the output voltages of the constant voltage power sources into line is also proposed, a stable supply of power may not be possible in the method if any of the constant voltage power sources breaks down, and the entire power system may not operate normally.
A power system including constant current power sources, in place of the constant voltage power sources, connected in parallel is also possible. In the power system, even if one of the constant current power sources breaks down, a stable supply of power is possible by using other constant current power sources. However, in the technique, there is a problem in which the output power of the power system is not stable.
Meanwhile, Patent Literature 2 and 3 describes techniques, in which only one of the power sources connected in parallel is a constant voltage power source, and the rest are constant current power sources. In the techniques, the output voltages of the constant current power sources are mixed with the output voltage of the constant voltage power source, and output currents in a preset size are output from the constant current power sources. As a result, while the output power of the power system is held constant, a stable supply of power is possible by using other constant current power sources even if one of the constant current power sources breaks down.